1. Field of the Invention
The present invention relates to a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a method for driving these devices, and a method for manufacturing these devices. The present invention particularly relates to a semiconductor device, a display device, a liquid crystal display device, and a light-emitting device which include a driver circuit formed over the same substrate as a pixel portion, and a method for driving these devices. Further, the present invention relates to an electronic device including the semiconductor device, the display device, the liquid crystal display device, and the light emitting device.
2. Description of the Related Art
In recent years, with the increase of large display devices such as liquid crystal televisions, display devices have been actively developed. In particular, a technique for forming a driver circuit such as a gate driver over the same substrate as a pixel portion by using transistors including a non-single-crystal semiconductor has been actively developed because the technique greatly contributes to reduction in cost and improvement in reliability.
In a transistor including a non-single-crystal semiconductor, degradation such as change in threshold voltage or reduction in mobility occurs. As such degradation of the transistor progresses, it becomes difficult to operate a driver circuit and incapable of displaying images. Patent Documents 1 and 2, and Non-patent Document 1 each disclose a shift register in which degradation of transistors (hereinafter, also referred to as a pull-down transistor) which include a function of maintaining an output signal of a flip flop in the Low level or a function of decrease the output signal to a Low level can be suppressed. In these documents, two pull-down transistors are used. The two pull-down transistors are connected between an output terminal of a flip flop and a wiring to which low power supply voltage (also referred to as a voltage Vss or negative power supply) is supplied. Moreover, one pull-down transistor and the other one pull-down transistor are alternately turned on (also referred to as an on state). Accordingly, the time during which the each pull-down transistor is on is reduced, so that degradation of characteristics of the one pull-down transistors can be suppressed.